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Bioregenerative processes for the replenishment of plant nutrients in CELSS are being evaluated. Continuous operation of a breadboard-scale aerobic bioreactor (breadboard-scale aerobic bioreactor, 120 L working volume) has been used successfully to resupply partially the nutrients required for hydroponically grown wheat (4 m2 growing area, 57 days bioreactor operation) and potato (8 m2 growing area, 310 days bioreactor operation). Bioreactor performance, measured by reduction in volatile solids (27 to 37% wheat, ca. 51% potato) and mineralization of biomass C (24 to 37% wheat, 35 to 61% potato), depended on process parameters such as retention time.

Ground studies are continuing at Kennedy Space Center to define the performance of the Immobilized Microbe Microgravity Water Processing System (IMMWPS), a denitrifying, fixed-bed reactor designed for shuttle flight-testing. The goal of these experiments was to define organic compounds that could be used as indicators of changes in reactor performance as to the removal of surfactant and to evaluate additional flight and sampling protocols. While changes in the breakthrough concentration of surfactant would provide insight into performance changes during the flight experiment, this breakthrough of surfactant in the flight system is undesirable due to operational problems resulting from foaming of the undegraded surfactant. By monitoring a degradation intermediate instead of the surfactant in the effluent, this problem could be avoided while monitoring any effects of microgravity on bioreactor performance during space flight.

NASA's Advanced Life Support Breadboard Project at Kennedy Space Center focuses on biological regeneration of essential commodities for long-term space missions. If plants are grown on these missions, roughly 50% of the biomass will be inedible. Composting can reduce the volume of inedible biomass, reduce levels of leachable soluble organics, and produce a mineral-rich leachate that can be used to provide nutrients to subsequent generations of plants. Other wastes will also be generated on space missions; co-composting of these wastes should increase the rate and extent of degradation and should assist in control of moisture content during composting. To investigate these assumptions, we added simulated human solid waste to freshly harvested inedible wheat biomass and composted the mixture for 21 days.

A Breadboard-Scale Aerobic Bioreactor (B-SAB) has been designed and integrated with the Kennedy Space Center's Biomass Production Chamber (BPC). The bioreactor utilizes a mixed microbial community to biodegrade inedible plant residues, a component of a Controlled Ecological Life Support System (CELSS) waste-stream. The continuously stirred tank reactor (120 L working volume) supports nutrient recycling and secondary food production experiments, and can process an influent with a solids loading as high as 50 g L-1. The volumetric oxygen mass transfer coefficient, kLa, is 0.013 s-1. Nutrient solution for BPC lettuce and wheat crops has been produced. Currently, B-SAB is supplying 80% of the nutrients for 10 m2 of potato plants in a continuous production experiment.

Regenerative life support systems are under development to reduce the need for resupply of essential commodities during long duration space missions. If higher plants are used to supply food, oxygen, and potable water, composters could be used to stabilize solid wastes, provide CO2 and nutrients to the plants, and achieve pathogen reduction. Small-scale aerobic composting was used successfully to degrade organic compounds in inedible potato biomass. Soluble nutrients were extracted from the compost at concentrations that supported seed germination. Further work is indicated to understand the inhibitory effects of some leachates. Future composter designs should allow improved performance through better instrumentation and process control.

Bioprocessing of human fecal wastes may be an important means for recycling of crop nutrients within a closed Advanced Life Support System. The objectives of this study were to determine the levels of key crop nutrients that can be extracted from human feces that had been autoclave sterilized vs. those that had not. When compared with inedible ALS grown wheat residues, the contribution of feces, which has an ash content 13% to the total potential, recoverable minerals may be small. This paper discusses results from bioreactor runs obtained using continuous stirred tank reactors with an 8 day batch culture of autoclaved or not autoclaved feces. The results suggest that feces should not be autoclaved if mineral recovery is desired. Biodegradation of feces ranged from 27 to 39% in 8 days, with 67 to 79% reduction in soluble total organic carbon (TOC) and concomitant production of carbon dioxide (CO2).